The burgeoning growth of terrestrial and satellite communications systems has been accompanied by the need to provide higher and higher information-carrying capacities within a limited frequency band. One of the techniques used to fulfill this need has been to simultaneously transmit independent data signals on carrier signals which have orthogonal polarizations and which occupy the allotted frequency band. This use of modulated, orthogonally-polarized carrier signals can double the information-carrying capacity of a communications link. However, time-varying distortion, such as rainfall, imperfect antenna alignment, etc., collectively known as fading, can interfere with the operation of carrier and timing recovery circuitry in the system receiver. Such circuitry is respectively used for demodulating the incoming carrier signal and then sampling the demodulated signal to reconstruct the data. At times, the fading is so severe that the carrier and timing recovery circuits are no longer synchronized to the incoming and the data signals can't be recovered.
One general class of techniques to reduce fading-induced data loss in systems which employ one or more carrier signal having a single polarization is to transmit the same data either over different line-of-sight routes or on a plurality of carrier signals having different frequencies. While these techniques, respectively known as spatial and frequency diversity, provide satisfactory results, their usage is not practical in certain applications and their cost can be prohibitive.
Other prior art attempts to minimize loss of the data signal caused by improper operation of carrier and timing recovery circuits have focussed on diminishing the acquisition time, i.e., the time required for such circuits to establish synchronization after synchronization has been lost due to severe distortion. While these attempts have been successful, there is still a loss of data during the acquisition time which exceeds the performance objectives of certain system applications.